The present invention relates to optical inspection of glassware containers and like translucent articles, and more particularly to optical inspection methods and apparatus for dimensional gauging of wall thickness and concentricity. The invention further relates to glassware inspection using fiber optic devices.
Thinwall detection is a critical type of dimensional gauging for glassware articles, inasmuch as thin areas within the walls of glass containers render the containers susceptible to breakage upon impact or pressurized filling. Most present inspection techniques for glass container thickness rely on contacting the container wall with capacitive detector probes. Sensitive electronic circuits measure frequency changes generated by the varying dielectric of glass as a capacitive element of the circuit. Such devices may employ linear probes which contact rolling containers, or may space the probe from the sidewall by a fixed distance during rotation.
Such capacitive measurement devices suffer several significant drawbacks. These devices do not perform well at high speeds of container rotation, at which the tracking head will not remain in stable engagement with the surface. When such tracking heads disengage the container sidewall, false thickness readings can occur. Such contact devices cannot effectively measure thickness at the container seams, and generally utilize a masking scheme in the electronics to ignore readings at or near the seams. This is unfortunate as thinwall distribution is often found at container seams. These contact devices are generally not well suited to measuring wall thickness at or near the heel or shoulder areas of containers. Furthermore, these prior art devices are subject to severe wear, and may cause scratching, chipping, or abrasion of container surfaces. Finally, this prior art technique is unsuitable for thinwall gauging of plastic containers, which are generally flexible and deform under the contact forces.
Thickness inspection has also played a significant role in other glass manufacturing processes, such as flat glass or transparent plastic sheet manufacture.
Contact inspection techniques have predominated as well in "out-of-round" measurements for glassware containers. One such prior art technique relies upon a spring or air loaded probe to track the container. Other commercial approaches include limit switches, and a combination of a mechanical tracking device defining a window with a laser beam directed through the window. Such mechanical approaches suffer many of the drawbacks recited above for prior art thinwall detectors.
U.S. Pat. No. 4,476,533 to Dandt et al. discloses an optical inspection system to be employed at the "hot end" of glassware container production shops. In the Dandt system, fiber optic bundles propogate light beams across the container paths on hot end conveyor lines. A sensing system positioned on the opposite side of the conveyor includes a focusing system and light detection devices. Electronic processing apparatus processes the photodetector outputs to determine various glassware dimensions. Various processing algorithms are disclosed for diameter, perpendicular offset, and roundness.
U.S. Pat. Nos. 3,327,584 to Kissinger and 3,940,608 to Kissinger et al. disclose fiber optic devises which are designed for measuring the proximity of a bifurcated fiber optic probe to a light-reflecting test object. These devices utilize adjacent pairs of light-transmitting and light-receiving fibers--the "fiber optic lever" principle. By the nature of the interaction between the field of illumination of the transmitting fibers, and the field of view of the detector fibers, such devices provide precisely varying output signals (intensity of received light) as a function of the gap between the fiber optic probe and the test surface. The later of these patents discloses the improvement wherein the probe incorporates a lens system to focus the image of the end face of the fiber optic bundle onto the test surface, and to refocus the reflected light onto the bundle end face in an upright relationship. These probes may be used to measure displacement, as well as to measure vibration characteristics, and Kissinger and his collaborators disclose a number of possible applications. Kissinger et al. do not, however, disclose the use of these devices in the detection of wall thickness of transparent containers, nor for concentricity measurements for round articles, either in the above patents or in other publications.
Accordingly, it is a primary object of the invention to provide noncontact inspection devices for the measurement of wall thickness of glassware containers and other transparent articles. Such devices should provide accurate, reliable use, without the mechanical shortcomings typical of contact measuring probes. This apparatus should be usable with containers having a variety of materials, dimensions, and other physical characteristics.
These devices should be capable of thinwall detection at container seams, and at the shoulders and heels.
A further object is to provide an improved technique for concentricity, or "out-of-round" measurement.
Yet another object is to achieve an inspection technique which may be easily adapted to translucent plastic articles.
A still further object is to provide an inspection technique which may be advantageously applied to other applications, such as the manufacturer of flat glass.